The invention relates generally to color processing and, more particularly, to techniques for color characterization and transformation.
Since the introduction of the CIE (Commission International de l""Eclairage) color measurement system in the early 1930""s, many different color spaces have been proposed for different applications. A color space, also referred to as a color xe2x80x9cmetric,xe2x80x9d is essentially a coordinate system by which a color can be quantified.
A color space can be used to characterize the color output of a color imaging system relative to other color imaging systems. By characterizing multiple color imaging systems, the color space facilitates using different imaging systems to produce matching colors. An xe2x80x9cidealxe2x80x9d color space would calculate a color mapping between different color imaging systems that achieves an acceptable color match between the systems without subjective or empirical adjustment.
Color spaces differ in the parameters expressed on their coordinate axes and the manner in which the parameters are calculated. CIE color spaces use CIE Standard Observer functions that are based on color matching functions and result in a unique set of tristimulus values XYZ for any color measured under specified conditions. The tristimulus values XYZ are calculated from the spectral output of either an additive or subtractive color system convoluted with the response function of either a 2 degree or 10 degree Standard Observer. In the case of reflective hard copy, the spectral reflectance curve is typically convoluted with a standard illuminant to estimate the expected spectral output of the reflective color.
One CIE color space is the CIELAB color space. In this color space, L* represents lightness, a* represents redness-greenness, and b* represents yellowness-blueness. The CIELAB color space employs a modified von Kries chromatic adaptation algorithm. According to the modified von Kries chromatic-adaptation transform, a description of which can be found in Gunter Wyszecki and W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, section 5.12, John Wiley and Sons, Inc., 1982, the L*a*b* color spaces make use of white reference tristimulus data. The modified von Kries chromatic-adaptation transform involves dividing the tristimulus values XYZ obtained for a color produced by a particular color imaging system by white reference tristimulus values for the system. For example, the X, Y, and Z tristimulus values of the color under study can be divided, respectively, by the X, Y, and Z tristimulus values for a perfectly diffuse white reflector. Thus the von Kries approach defines both neutral and chromatic colors relative to the xe2x80x9cwhite referencexe2x80x9d representing the XYZ tristimulus values of the perfectly diffuse white reflector. The equations for the CIE 1976 CIELAB color space are as follows:                               L          *                =                              116            xc3x97                          f              ⁡                              (                                  Y                  /                                      Y                    n                                                  )                                              -          16                                    [1]                                          a          *                =                  500          xc3x97                      [                                          f                ⁡                                  (                                      X                    /                                          X                      n                                                        )                                            -                              f                ⁡                                  (                                      Y                    /                                          Y                      n                                                        )                                                      ]                                              [2]                                          b          *                =                  200          xc3x97                      [                                          f                ⁡                                  (                                      Y                    /                                          Y                      n                                                        )                                            -                              f                ⁡                                  (                                      Z                    /                                          Z                      n                                                        )                                                      ]                                              [3]                                          xe2x80x83                ⁢                                                                                                  f                    ⁡                                          (                      ω                      )                                                        =                                                            (                      ω                      )                                                              1                      /                      3                                                                                                  ω                                               greater than           0.008856                ⁢                  xe2x80x83                                    [4]                                          xe2x80x83                ⁢                                                                              f                  ⁡                                      (                    ω                    )                                                  =                                                      7.787                    ⁢                                          (                      ω                      )                                                        +                                      16                    /                    116                                                                                                      ω                ≤                0.008856                                                    ⁢                  xe2x80x83                                    [5]            
where Xn, Yn, and Zn, are the tristimulus values of a perfectly diffuse white reflector under specified viewing conditions. The viewing conditions are determined by (1) the illuminant, e.g., D50, and (2) the Standard Observer (2xc2x0 or 10xc2x0).
In general, in one aspect, the invention provides a method of characterizing a color imaging system. The method comprises obtaining first data indicative of output of the color imaging system. The first data is processed, to yield second data, according to a color appearance model that varies in accordance with neutrality of colors indicated by the first data.
In general, in another aspect, the invention provides a computer program product residing on a computer readable medium, for characterizing a color imaging system, and comprising instructions. The instructions are for causing a computer to obtain first data indicative of output of the color imaging system and to process the first data, to yield second data, according to a color appearance model that varies in accordance with neutrality of a color indicated by the first data.
In general, in another aspect, the invention provides a method of producing a color on a device. The method comprises obtaining first data associated with a first device and indicative of a first color. Second data are determined that are related to stimulus data of the first device by a color appearance model that converts input data to output data using a white reference vector that varies in association with a neutrality of a color indicated by the input data. A second device is actuated according to the second data to produce a second color to approximate the first color.
In general, in another aspect, the invention provides a computer program product residing on a computer readable medium, for producing a color on a device, and comprising instructions. The instructions are for causing a computer to obtain first data associated with a first device and indicative of a first color. The instructions are also for causing the computer to determine second data related to stimulus data of the first device by a color appearance model that converts input data to output data using a white reference vector that varies in association with a neutrality of a color indicated by the input data. The instructions are also for causing the computer to actuate a second device according to the second data to produce a second color to approximate the first color.
In general, in another aspect, the invention provides a method of producing a color with an emissive device using absolute colorimetry. The method comprises obtaining first data indicative of a first color. Second data are determined that are related to the first data by a color appearance model that uses a white point of the emissive device as a white reference vector. The emissive device is actuated according to the second data to implement absolute colorimetry to produce a second color to approximate the first color.
In general, in another aspect, the invention provides a computer program product residing on a computer readable medium, for producing a color with an emissive device using absolute colorimetry, and comprising instructions. The instructions are for causing a computer to obtain first data indicative of a first color and to determine second data related to the first data by a color appearance model that uses a white point of the emissive device as a white reference vector. The instructions are also for causing the computer to actuate the emissive device according to the second data to implement absolute colorimetry to produce a second color to approximate the first color.
In general, in another aspect, the invention provides a method of characterizing an emissive device for absolute colorimetry. The method comprises obtaining first data indicative of output of the emissive device. The first data are converted to second data using a color appearance model that uses a white point of the emissive device as a reference white vector. The second data are provided for use in absolute calorimetric color reproduction.
In general, in another aspect, the invention provides a computer program product residing on a computer readable medium, for characterizing an emissive device for absolute colorimetry, and comprising instructions. The instructions are for causing a computer to obtain first data indicative of output of the emissive device and to convert the first data to second data using a color space that uses a white point of the emissive device as a reference white vector. The instructions are also for causing the computer to provide the second data for use in absolute calorimetric color reproduction.
In general, in another aspect, the invention provides a method of characterizing colors for reproduction between a first device and a second device. The method includes normalizing first tristimulus values indicative of a color of the first device using local black point values. The normalized first tristimulus values are transformed to obtain color values indicative of modified cone responses of the human eye. The color values are chromatically adapted from a local condition to a reference condition. The adapted color values are transformed to obtain second tristimulus values.
Embodiments of the invention may provide one or more of the following advantages. The invention provides more accurate color characterization over the entire range of a color imaging system than color characterization using a fixed white reference point. The helps ensure substantial uniformity of the color characterization for both light color shades and more intense colors, and for neutral and non-neutral colors. Emissive devices can be caused to display colors, both saturated and neutral, that match well to corresponding colors on absorptive devices.